Tungsten Alloy for Shaped Charge Liners
A shaped charge liner is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, initiate nuclear weapons, and penetrate armor. A typical modern lined shaped charge can penetrate armor steel to a depth of seven or more times the diameter of the charge's cone (cone diameters, CD), though greater depths of 10 CD and above are now feasible.
The shape most commonly used for the liner is a cone, with an internal apex angle of 40 to 90 degrees. Different apex angles yield different distributions of jet mass and velocity. Small apex angles can result in jet bifurcation, or even in the failure of the jet to form at all; this is attributed to the collapse velocity being above a certain threshold, normally slightly higher than the liner material's bulk sound speed. Other widely used shapes include hemispheres, tulips, trumpets, ellipses, and bi-conics; the various shapes yield jets with different velocity and mass distributions.
Liners have been made from many materials, including glass and various metals. The deepest penetrations are achieved with a dense, ductile metal, and a very common choice has been copper. For some modern anti-armor weapons, molybdenum and pseudo-alloys of tungsten filler and copper binder (9:1 thus density is ~18t/m3) have been adopted. Just about every common metallic element has been tried, including aluminum, tungsten, tantalum, depleted uranium, lead, tin, cadmium, cobalt, magnesium, titanium, zinc, zirconium, molybdenum, and beryllium, nickel, silver, and even gold and platinum. The selection of the material depends on the target to be penetrated; for example, aluminum has been found advantageous for concrete targets.
Why Choose Tungsten Alloy as Shaped Charge Liners?
For the deepest penetrations, pure metals yield the best results, because they display the greatest ductility, hence postponing the breakup of the stretching jet into particles. In charges for oil-well completion, however, tungsten alloy shaped charge liner is essential that a solid slug or "carrot" not be formed, since it would plug the hole just penetrated and interfere with the influx of oil. In the petroleum industry, therefore, liners are generally fabricated by powder metallurgy, often of pseudo-alloys, which if tungsten alloy shaped charge liners un-sintered, yield jets that are composed mainly of dispersed fine metal particles.
During World War II, shaped charge liners were made of copper or steel, though other materials were tried or researched. The precision of the charge's construction and shaped charge liners detonation mode were both inferior to modern warheads. This lower precision caused the jet to curve and to break up at an earlier time and hence at a shorter distance. The resulting dispersion decreased the penetration depth for a given cone diameter and also shortened the optimum standoff distance. Since the charges were less effective at larger standoffs, side and turret skirts fitted to some German tanks to protect against Russian anti-tank rifle fire were fortuitously found to give the jet room to disperse and hence reduce its penetrating ability. Now most shaped charge liners are made of tungsten alloy.
The use of shaped charge liners today may increase the penetration of some warheads. Due to constraints in the length of the projectile/missile, the built in stand-off on many warheads is not the optimum distance. The skirting effectively increases the distance between the amour and the target, providing the warhead with a more optimum standoff and greater penetration if the optimum stand-off is not drastically exceeded. Tungsten alloy shaped charge liners should not be confused with cage amour that is used to damage the fusing system of RPG-7 projectiles. The amour works by deforming the inner and outer orgies and shorting the firing circuit between the rocket's piezoelectric nose probe and rear fuse assembly. If the nose probe strikes the amour, the warhead will function as normal.
The spacing between the shaped charge liners and shaped charge liners target is critical, as there is an optimum standoff distance to achieve the deepest penetration. At short standoffs, the jet does not have room to stretch out, and at long standoffs, shaped charge liners eventually breaks into particles, which then tend to drift off the shaped charge liners of axis and to tumble, so that the successive particles tend to widen rather than deepen the hole. At very long standoffs, velocity is lost to air drag, degrading penetration further.
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